π Order Book Order Book
A live, always-updating list of every open buy and sell order for a trading pair on an exchange. It shows the price and quantity people want to trade at, and a trade happens the moment a buyer's price and a seller's price line up.
π§Ί The simple version β a farmers'-market bulletin board
Picture a bulletin board at a market. One column lists every buyer's note: 'I'll pay up to $X for this.' The other column lists every seller's note: 'I'll sell mine for $Z.' The order book is exactly that, but for a coin, and it refreshes constantly. The buy side is called bids and the sell side is called asks. A sale happens the instant a buyer's price and a seller's price line up.
βοΈ Bids, asks, and the spread
Bids are sorted highest-to-lowest, so the buyer willing to pay the most sits at the top. Asks are sorted lowest-to-highest, so the cheapest seller sits at the top. Those two top prices face each other, and the gap between them is the bid-ask spread. A narrow spread points to a busy, liquid market; a wide one warns that your fill price could drift, a problem known as slippage.
| What you see | What it tells you |
|---|---|
| π’ Best bid | The highest price anyone is currently willing to pay |
| π΄ Best ask | The lowest price anyone is currently willing to sell at |
| βοΈ Small spread | Lots of activity and high liquidity β easy to buy or sell near the shown price |
| πͺ« Wide spread | A thin, illiquid market β your trade can move the price against you |
βοΈ How a trade actually fills
When the highest bid meets the lowest ask, the exchange's matching engine pairs them automatically. The filled orders are removed from the book, and new ones keep arriving. That is why the order book never sits still. The order types that interact with it:
- β‘ Market order β fills right away at the best price available on the book
- π― Limit order β fills only at the price you set, or better; until then it waits in the book
- π Stop order β sits idle until the price hits a threshold you chose, then activates
π Market depth β what it would cost to move the price
Stack up all the orders sitting at each price level and you get market depth, often drawn as a depth chart. It shows how much volume is waiting nearby, and therefore how far the price would slide if a large order ate through it. Deep books absorb big trades calmly; shallow books lurch.
π’π΄ Where a beginner meets it
Open the trading screen on a centralized exchange like Binance, Coinbase or Kraken and the order book is the green-and-red ladder next to the price chart. It is the core price-discovery mechanism: the visible record of supply meeting demand. Many decentralized exchanges skip it and use an automated market maker instead, though a few like Injective still run an order book on-chain.
β FAQ
- What is the bid-ask spread?
- It's the gap between the highest price a buyer will pay (best bid) and the lowest price a seller will accept (best ask). A small spread usually means lots of trading activity and easy buying or selling. A wide spread means a thin market where your trade can move the price.
- Does a big wall of orders mean the price will move that way?
- Not necessarily. Large buy or sell walls can create a false impression of demand or supply, and they can be cancelled before they ever fill. Treat the order book as one signal among several, not a promise about where the price is headed.
- Do all crypto exchanges use an order book?
- Most centralized exchanges like Binance, Coinbase and Kraken do. Many decentralized exchanges instead use an automated market maker with liquidity pools and a pricing formula. A few decentralized exchanges, such as Injective and dYdX, still run on an order book model.